Cell Identities (IDs) are used in various wireless communication systems to identify cells. For example, in a wireless communication system using Long Term Evolution (LTE) technology, the physical cell identity (PCI) is assigned to and transmitted by all LTE cells. It is a layer 1 radio signature of which there exists 504 unique data sequences, and serves as a cell signature that is detectable for User Equipments (UEs) connectable to the wireless communication system. UEs use the PCI to determine and assess the relation to the serving cell and to identify neighboring cells. The PCIs are geographically reused in the network to avoid conflict. If two cells geographically close to each other use the same PCI, the UEs will have difficulties to identify them, and as a result, serious interference will occur.
Various cell ID allocation methods have been proposed and as a principle the conflict in cell IDs should be avoided. However, in some scenarios the conflict appears to be more difficult to avoid. One typical scenario is the “mobile” base station in vehicles such as a train.
Due to the high demand for mobile wireless services such as online game and Internet surfing especially in a long time travelling, a reliable and high data rate wireless service is necessary for train passengers. Typically at least one mobile base station is deployed on the top of carriages to implement wireless backhaul to base stations along with the railway. This is similar to the femtocell, which is a home base station for indoor coverage extension and wideband data service. However, unlike the femtocell, the mobile base station moves with the train. It is possible that the mobile base station allocated with a cell ID happens to travel through an area covered by a base station allocated with the same cell ID. In this case, the mobile base station uses the same cell ID and in turn the same wireless resource as the outside base station does. Since the signal penetration loss from inside carriages to outside carriages is not enough to avoid interference to the outside base station and vice versa, resource conflict occurs inevitably, which may result in wireless service breaking off. The chance that cell ID conflict occurs will be high if the train travels through a long distance.
One solution for the problem is to reserve certain cell IDs for trains. However, considering the limited number of available cell IDs, e.g. 504 PCIs in LTE, the solution is almost infeasible.